At the end of next month (September 2021) the root CA certificate for DST Root CA X3 expires.
When a new CA comes into existence in the Web PKI they face a significant problem, even if all the brand new shiny software trusts their excellent new CA root, almost everybody at least sometimes needs old software that might not have the latest updates. So, they get an existing trusted CA to sign to say that they trust this new CA.
The charity that provides Let's Encrypt, ISRG, has a root CA named ISRG Root X1 (and a newer one named ISRG Root X2 but that's another matter) but they also sought such cross signatures from IdenTrust, who had bought the Digital Signature Trust and with it DST Root CA X3 in order to "bootstrap" trust on older systems.
So up until late September, a device that has no idea who ISRG are or what Let's Encrypt is, still trusts Let's Encrypt certificates, because it trusts DST Root CA X3. And then the self-signed certificate for DST Root CA X3 expires.
Tools like RunAsDate allow you to test your software to see whether it will still work after that expiry date. It wasn't really practical to run such tests a few months ago, because if in say May you tell the computer it's now the 1st of October all your perfectly good Let's Encrypt certificates issued in April have expired by October, so that's why nothing works. However, since early July it has been possible to have certificates that will not be expired when DST Root CA X3 expires and perform such tests.
Lots of maintainers probably look after systems where they can't (or at least, daren't) add a newer root like ISRG Root X1 and would value knowing in advance that the system will blow up completely at the end of September, rather than being blind-sided when it inevitably happens. RunAsDate (and similar tools on other systems) enable them to find that out with a few weeks left to fix or at least mitigate the problem.